Defective pixel compensation method

ABSTRACT

The present invention relates to a method for compensating the impact of at least one defective pixel with a known position in a spatial light modulator (SLM) when creating a pattern of the SLM on a work piece covered with a layer sensitive to electromagnetic radiation. A source for emitting electromagnetic radiation is provided. Said radiation is illuminating said SLM having a plurality of modulating elements (pixels). In a writing pass an image of said modulator is projected on said work piece. A compensation for defective pixels in at least one other writing pass is performed. The invention also relates to an apparatus for performing said method.

TECHNICAL FIELD

[0001] The present invention relates in general to techniques forobtaining improved images by compensation methods. In particular itrelates to a method for compensating defective pixels in a Spatial LightModulator (SLM), used in optical lithography. It also relates to anapparatus for patterning a work piece comprising such a method and amethod for detecting defective pixels.

DESCRIPTION OF THE BACKGROUND ART

[0002] Lithographic production is useful for integrated circuits, masks,reticles, flat panel displays, micro-mechanical or micro-optical devicesand packaging devices, e.g. lead frames and MCM's. Lithographicproduction may involve an optical system to image a master pattern froma computer-controlled reticle onto a workpiece. A suitable workpiece maycomprise a layer sensitive to electromagnetic radiation, for examplevisible or non-visible light. An example of such a system is describedin WO 9945435 with the same inventor and applicant as the presentinvention.

[0003] Said computer controlled reticle may be a Spatial Light Modulator(SLM) comprising a one or two dimensional array or matrix of reflectivemovable micro mirrors, a one or two dimensional array or matrix oftransmissive LCD crystals, or other similar programmable one- or twodimensional arrays based on gratings effects, interference effects ormechanical elements (e.g., shutters).

[0004] In general, these computer controlled reticles may be used forthe formation of images in a variety of ways. These reticles, such as anSLM, include many modulating elements or pixels, in some instancesmillion or more pixels. For example a problem with Spatial LightModulators is that one or a plurality of pixels in a given SLM may bedefective, i.e. they may not respond to a control signal as intended.

[0005] These defective pixels in a computer controlled reticle maylimiting the resolution and accuracy available for their use in opticalimaging; e.g., the production of printed patterns on a workpiece may belimited as to its line widths and accuracy.

[0006] Therefore, there is a need in the art for a method, whicheffectively and precisely finds and compensates for defective pixels inthe SLM.

SUMMARY OF THE INVENTION

[0007] In view of the foregoing background, the compensation fordefective pixels in the SLM, such as for example a mirror elements stuckin a specific position, is useful to form images having sub micron linewidths with tolerances approaching 5 nm.

[0008] Accordingly, it is an object of the present invention to improvethe images formed using spatial light modulators by providing animproved method for the compensation of defective pixels.

[0009] In a first embodiment, the invention provides a method forcompensating the impact of at least one defective pixel with a knownposition in a spatial light modulator (SLM) when creating a pattern ofthe SLM on a work piece covered with a layer sensitive to aselectromagnetic radiation. Said method comprising the actions ofproviding a source for emitting electromagnetic radiation, illuminatingby said radiation said SLM having a plurality of modulating elements(pixels), projecting in a writing pass an image of said modulator onsaid work piece, and performing a compensation for defective pixels inat least one other writing pass.

[0010] In another embodiment of the invention said electromagneticradiation is a pulsed laser source.

[0011] In another embodiment of the invention a single defective pixelin one writing pass is compensated with a single compensating pixel inanother writing pass.

[0012] In another embodiment of the invention a single defective pixelin one writing pass is compensated with a plurality of compensatingpixel in another writing pass.

[0013] In another embodiment of the invention said SLM is illuminated bya radiation dose in the different writing passes.

[0014] In another embodiment of the invention said SLM is illuminated bydifferent radiation intensities in the different writing passes.

[0015] In another embodiment of the invention said SLM is a transmissiveSpatial Light Modulator.

[0016] In another embodiment of the invention said SLM is a reflectiveSpatial Light Modulator.

[0017] In another embodiment of the invention the pixels in said SLM isoperated in an analog manner.

[0018] The invention relates also to a method for compensating theimpact of at least one defective pixel with a known position in aspatial light modulator (SLM) when creating a pattern of the SLM on awork piece covered with a layer sensitive to electromagnetic radiation.Said method comprising the actions of, providing a source for emittingelectromagnetic radiation, illuminating by said radiation said SLMhaving a plurality of modulating elements (pixels), projecting an imageof said SLM on the detector arrangement for measuring a dose ofradiation, and performing a compensation of said defective pixel by atleast one of the most adjacent pixels in the SLM.

[0019] In another embodiment of the invention said compensation isperformed by assigning said at least one of the most adjacent pixels bya value given by subtraction of an intended pixel value by a actualpixel value divided by the number of most adjacent pixels used forcompensation.

[0020] The invention relates also to a method for compensating theimpact of at least one defective pixel in a spatial light modulator(SLM) when creating a pattern of the SLM on a work piece covered atleast partially with a layer sensitive to electromagnetic radiation.Said method comprising the actions of, setting the pixels in said SLM ina predetermined state, illuminating by a radiation source said SLM,projecting the image of the SLM onto the detector arrangement thatmeasures dose of the SLM pixels, identifying defective pixels, andperforming a compensation for said defective pixels in at least onewriting pass.

[0021] The invention relates also to a method for compensating theimpact of at least one defective pixel with a known position in aspatial light modulator (SLM) when creating a pattern of the SLM on awork piece covered with a layer sensitive to electromagnetic radiation.Said method comprising the actions of, providing a source for emittingelectromagnetic radiation, illuminating by said radiation said SLMhaving a plurality of modulating elements (pixels), projecting in afirst writing pass an image of said modulator on said work piece using afirst set of pixels in said SLM, performing a pre compensation fordefective pixels in at least one subsequent writing pass in at least oneprior writing pass, and projecting in at least a second writing passsaid image of said modulator on said work piece using at least a secondset of pixels in said SLM.

[0022] In another embodiment of the invention, said method furthercomprising the action of, performing a post compensation for defectivepixels in at least one prior writing step in at least one subsequentwriting pass.

[0023] In another embodiment of the invention a post compensation fordefective pixels in at least one prior writing step in at least onesubsequent writing pass is performed instead of said pre compensation.

[0024] In another embodiment of the invention said electromagneticradiation is a pulsed laser source.

[0025] In another embodiment of the invention, said method furthercomprising the action of, including at least one pixel in said first setof pixels in said at least second set of pixels.

[0026] In another embodiment of the invention a single defective pixelin one writing pass is compensated with a single compensating pixel inanother writing pass.

[0027] In another embodiment of the invention a single defective pixelin one writing pass is compensated with a plurality of compensatingpixels in another writing pass.

[0028] In another embodiment of the invention said SLM is illuminated bythe same radiation dose in different writing passes.

[0029] In another embodiment of the invention said SLM is illuminated bydifferent radiation dose in different writing passes.

[0030] In another embodiment of the invention said SLM is a transmissiveSpatial Light Modulator.

[0031] In another embodiment of the invention said SLM is a reflectiveSpatial Light Modulator.

[0032] In another embodiment of the invention the pixels in said SLM isoperated in an analog manner.

[0033] In another embodiment of the invention an image of said pixels insaid first writing pass is displaced in said SLM relative said image ofsaid pixels in said second writing pass by one or a plurality of pixels.

[0034] In another embodiment of the invention an image of said pixels insaid first writing pass is displaced on said workpiece relative saidimage of said pixels in said second writing pass by at least a fractionof a pixel.

[0035] In another embodiment of the invention said first set of pixelsbelongs to a first SLM and said second set of pixels belong to a secondSLM.

[0036] In another embodiment of the invention said first and second SLMsare illuminated simultaneously.

[0037] In another embodiment of the invention said first and second SLMsare illuminated by different radiation intensities.

[0038] The invention relates also to an apparatus for compensating theimpact of at least one defective pixel with a known position in aspatial light modulator (SLM) when creating a pattern of the SLM on awork piece covered with a layer sensitive to electromagnetic radiation,comprising a source for emitting electromagnetic radiation, a projectionsystem to project in a first writing pass an image of said modulator onsaid work piece using a first set of pixels in said SLM, means forperforming a pre compensation of defective pixels in at least onesubsequent writing pass in at least one prior writing pass, a projectionsystem to project in at least a second writing pass said image of saidmodulator on said work piece using at least a second set of pixels insaid SLM, means for performing a post compensation of defective pixelsin at least one prior writing pass in at least one latter writing pass.

[0039] In another embodiment of the invention said electromagneticradiation is a pulsed laser source.

[0040] In another embodiment of the invention at least one pixel in saidfirst set of pixels is included in said at feast a second set of pixels.

[0041] In another embodiment of the invention said projection system toproject in at least a second writing pass comprises, said SLMreprogrammed with the image to be written on said work piece with saidat least second set of pixels, a movable stage upon which stage saidwork piece is arranged in order to illuminate the same feature on saidwork piece with said at least second writing pass as said first writingpass.

[0042] In another embodiment of the invention said movable stage ismoved the length of N SLM pixels.

[0043] In another embodiment of the invention said stage is moved alonga row of pixels.

[0044] In another embodiment of the invention said movable stage ismoved along a column of pixels.

[0045] In another embodiment of the invention said movable stage ismoved along both a row of pixels and a column of pixels.

[0046] In another embodiment of the invention said movable stage ismoved the length of N SLM pixels plus a fraction of a SLM pixel.

[0047] In another embodiment of the invention a single defective pixelin one writing pass is compensated with a single compensating pixel inanother writing pass.

[0048] In another embodiment of the invention a single defective pixelin one writing pass is compensated with a plurality of compensatingpixel in another writing pass.

[0049] In another embodiment of the invention said SLM is illuminated bya same radiation dose in the different writing passes.

[0050] In another embodiment of the invention said SLM is illuminated bydifferent radiation intensities in the different writing passes.

[0051] In another embodiment of the invention said SLM is a transmissiveSpatial Light Modulator.

[0052] In another embodiment of the invention said SLM is a reflectiveSpatial Light Modulator.

[0053] In another embodiment of the invention the pixels in said SLM isoperated in an analog manner.

[0054] The invention relates also to an apparatus for compensating theimpact of at least one defective pixel with a known position in aspatial light modulator (SLM) when creating a pattern of the SLM on awork piece covered with a layer sensitive to electromagnetic radiation,comprising a source for emitting electromagnetic radiation, a projectionsystem for illuminating said SLM, having a plurality of modulatingelements (pixels), by said radiation and projecting in a writing pass animage of said modulator on said work piece, the detector arrangement(65) for measuring the dose of pixels from the image of the SLM and acomputer (66) for performing a compensation for defective pixels in atleast one other writing pass out of said image on said detector (65).

[0055] The invention relates also to an apparatus for compensating theimpact of at least one defective pixel with a known position in aspatial light modulator (SLM) (30) when creating a pattern of the SLM(30) on a work piece (60) covered with a layer sensitive toelectromagnetic radiation, comprising a source for emittingelectromagnetic radiation, a projection system for illuminating said SLM(30), having a plurality of modulating elements (pixels), by saidradiation and projecting in a writing pass an image of said modulator(30) on said work piece (60), the detector arrangement (65) formeasuring the dose of pixels from the image of the SLM, and a computer(66) for performing a compensation for defective pixels (110) by usingat least one of the most adjacent pixels (111, 112, 113, 114, 115, 116,117, 118) to said defective pixel (110).

[0056] In another embodiment of the invention the pixel intensities aredetected by said detector (65) whenever a new work piece (60) is to bepatterned.

[0057] The invention also relates to a method for detecting at least onedefective pixel in at least one SLM. Said method comprising the actionsof addressing all pixels in said at least one SLM with a first steeringsignal, illuminating said at least one SLM with electromagneticradiation, recording an image of said at least one SLM, computing agradient field of the recorded image, computing a divergence of thegradient field, identifying extreme values from the computed divergencewhich corresponds to defective pixels.

[0058] In another embodiment said invention further comprising theactions of addressing all pixels said at least one SLM with a secondsteering signal, illuminating said at least one SLM with electromagneticradiation, recording an image of said at least one SLM, computing agradient field of the recorded image, computing a divergence of thegradient field, identifying extreme values from the computed divergence,where defective pixels corresponds to extreme values from said firststeering signal and said second steering signal representing samepixels.

[0059] The invention also relates to a method for detecting at least onedefective pixel in at least one SLM. Said method comprising the actionsof addressing all pixels in said at least one SLM with a first steeringsignal, illuminating said at least one SLM with electromagneticradiation, recording a first image of said at least one SLM, addressingall pixels in said at least one SLM with a second steering signal,illuminating said at least one SLM with electromagnetic radiation,recording a second image of said at least one SLM, computing thedifference between said first image and said second image, identifyingbad pixels where the computed difference has a local minimum.

[0060] The invention also relates to a method for detecting at least onedefective pixel in at least one SLM Said method comprising the actionsof addressing a pattern to said at least one SLM, illuminating said SLMwith electromagnetic radiation, recording a first image of said at leastone SLM, comparing said recorded image with pattern data at featureedges, identifying bad pixels where the feature edge is moved apredetermined distance.

[0061] In another embodiment said pattern is a chessboard pattern.

[0062] In another embodiment said pattern is a pattern with parallellines.

[0063] In still another embodiment said method further comprising theaction of addressing said pattern with another set of pixels in said atleast one SLM, illuminating said SLM with electromagnetic radiation,recording a second image of said at least one SLM, comparing saidrecorded second image with pattern data at feature edges, identifyingbad pixels where the feature edge is moved a predetermined distance.

[0064] In still another embodiment said method further comprising theaction of comparing the feature edge movement in said first image withsaid second image for identifying bad pixels stuck at intermediatevalues.

[0065] In yet another embodiment said different writing passes isperformed by means of one SLM.

[0066] In yet another embodiment said different areas of said SLM areused in the different writing passes.

[0067] In yet another embodiment said different writing passes isperformed by means of a plurality of SLMs.

[0068] In yet another embodiment said different writing passes isperformed by means of one SLM.

[0069] In yet another embodiment said different areas of said SLM areused in the different writing passes.

[0070] In yet another embodiment said different writing passes isperformed by means of a plurality of SLMs.

BRIEF DESCRIPTION OF THE DRAWINGS

[0071] For a more complete understanding of the present invention, andthe advantages thereof, reference is now made to the followingdescription taken in conjunction with the accompanying drawings, inwhich:

[0072]FIG. 1 illustrates a portion of the top view of an array of pixelsin a Spatial Light Modulator (SLM) comprising a defective pixel.

[0073]FIG. 2 illustrates a defective pixel in two overlaid writingpasses with displaced fields.

[0074]FIG. 3 illustrates a view of the principal components in anoptical lithography system using an SLM, which may use the inventivemethod.

[0075]FIG. 4 illustrates a portion of the top view of an array of pixelsin a Spatial Light Modulator (SLM) comprising a defective pixel andcompensating pixels.

[0076]FIG. 5 illustrates the relationship between the pixels in thedetector arrangement and the dose distribution of the image produced byfour pixels of an SLM.

[0077]FIG. 6 illustrates the relationship between the control signalapplied to a pixel of an SLM and the resulting energy andelectromagnetic field amplitude.

[0078]FIG. 7 illustrates a flowchart of one calibration method.

[0079]FIG. 8 depicts a section view of SLM pixels without controlsignals.

[0080]FIG. 9 depicts the same pixels with control signals, but withoutpixel calibration.

[0081]FIG. 10 illustrates the same pixels and control signals as FIG.2b, but with pixel calibration.

[0082]FIG. 11 illustrates the response of a portion of the CCD to theuncompensated image produced by an uncalibrated SLM.

[0083]FIG. 12 illustrates a plurality of dose responses as a function ofthe control signals applied to their respective pixels.

[0084]FIG. 13 illustrates a possible relationship between the controlsignal applied to a pixel and the detected dose.

[0085]FIG. 14 illustrates an overview of the data path.

[0086]FIG. 15 illustrates another embodiment of the invention.

[0087]FIG. 16 illustrates an alternative setup of the pattern generator.

DETAILED DESCRIPTION OF THE PREFFERED EMBODIMENTS

[0088] Workpiece in the description below is meant to mean one of thegroup of: substrate for producing semiconductors (direct write), masksubstrate, reticle substrate.

[0089]FIG. 3 illustrates an exemplary embodiment of an apparatus 1 forpatterning a work piece 60. Said apparatus 1 comprising a source 10 foremitting electromagnetic radiation, a first lens arrangement 50, acomputer controlled reticle 30, a beam conditioner arrangement 20, aspatial filter 70 in a Fourier plane, a third lens arrangement 40, asecond lens arrangement 45, a beam splitter 90 and a detectorarrangement 65, a computer 66.

[0090] The source 10 may emit radiation in the range of wavelength frominfrared (IR), which is defined as 780 nm up to about 20 nm, to extremeultraviolet (EUV), which in this application is defined as the rangefrom 100 nm and down as far as the radiation is possible to be treatedas electromagnetic radiation, i.e. reflected and focused by opticalcomponents. The source 10 emits radiation either pulsed or continuously.The emitted radiation from the continuous radiation source 10 can beformed into a pulsed radiation by means of a shutter located in theradiation path between said radiation source 10 and said computercontrolled reticle 30. As an example can the radiation source 10, i.e.the source of an exposure beam, may be a KrF excimer laser with a pulsedoutput at 248 nm, a pulse length of approximately 10 ns and a repetitionrate of 1000 Hz. The repetition rate may be below or above 1000 Hz.

[0091] Not shown in FIG. 3 is an aperture located between the radiationsource and the SLM. The size of the aperture in combination with theFourier aperture may be changed in order to increase/decrease theresolution on the workpiece with constant σ.

[0092] The beam conditioner arrangement may be a simple lens, anassembly of lenses, and/or other optical components. The beamconditioner arrangement 20 distributes the radiation emitted from theradiation source 10 uniformly over at least a part of the surface of thecomputer-controlled reticle 30. In a case of a continuous radiationsource a beam of such a source may be scanned over the surface of thecomputer controlled reticle.

[0093] Between the radiation source 10 and the computer-controlledreticle 30, which may for instance be a spatial light modulator (SLM),said beam conditioner arrangement is arranged, which unit 20 expand andshapes the beam to illuminate the surface of the SLM in a uniformmanner. In a preferred embodiment with an excimer laser as the sourcethe beam shape is rectangular, the beam divergence different inx-direction and Y-direction and the radiation dose is often non-uniformover the beam cross-section. The beam may have the shape and size of theSLM 30 and homogenized so that the rather unpredictable beam profile isconverted to a flat illumination with a uniformity of, for example,1-2%. This may be done in steps: a first beam shaping step, ahomogenizing step and a second beam-shaping step. The beam is alsoangularly filtered and shaped so that the radiation impinging on eachpoint on the SLM has a controlled angular sub tense.

[0094] The optics of the invention is similar to that of a waferstepper. In steppers, the beam is homogenized in a light pipe. Saidlight pipe might be a rectangular or prism-shaped rod with reflectinginternal walls, where many mirror images of the light source are formed,so that the illumination is a superposition of many independent sources.Splitting and recombining the beam by refractive, reflective ordiffractive optical components might also perform the homogenization.

[0095] The electromagnetic radiation is directed towards the detectorarrangement that measures the dose of electromagnetic radiation, whichmay comprise a Charged Coupled Device (CCD) camera, a MOS-camera, or aCharged Injection Device (CID). The first lens arrangement 50 playsmainly the same role as the second lens arrangement 45, namely to createan identical image of the SLM surface on the work piece 60.

[0096] The SLM 30 and the detector arrangement for measuring dose ofelectromagnetic radiation 65 are connected to a control device 66, whichfor example can be a personal computer. The computer keeps track ofdefective pixels and compensates for the defective pixels according toinventive methods described herein below.

[0097]FIG. 8 illustrates one dimension from the array of pixels 200 inthe Spatial Light Modulator (SLM) as shown in FIG. 1. In this embodimentthe pixels 200 comprises movable micro mirrors 10, 11, 12, 13, 14, 15,16 said pixels being arranged movably coupled to a substrate 300comprising support members 310, 311, 312, 313, 314, 315, 316 for saidmovable micro mirrors 10, 11, 12, 13, 14, 15, 16 and address electrodes410, 411, 412, 413, 414, 415, 416.

[0098] By applying a first control signal, e.g. a first voltage, on saidaddress electrodes 410, 411, 412, 413, 414, 415, 416 and a secondcontrol signal, e.g. a second voltage, on said movable micro mirrors 10,11, 12, 13, 14, 15, 16, said micro mirrors 10, 11, 12, 13, 14, 15, 16may deflect around a deflection axis defined by a hinge arranged(coupled?) to the support members 310, 311, 312, 313, 314, 315, 316. Thedegree of deflection of each of the micro mirrors will be related to thesignal differential, e.g., voltage differential, between said addresselectrodes 410, 411, 412, 413, 414, 415, 416 and said movable micromirrors 10, 11, 10 12, 13, 14, 15, 16. The view shown in FIG. 2a mayrepresent (slightly exaggerated for clarification) an electrostaticallyunattracted state in which no voltage is applied to the addresselectrodes 410, 411, 412, 413, 414, 415, 416 or the movable micromirrors 10, 11, 12, 13, 14, 15, 16.

[0099]FIG. 8 illustrates a random deflection arrangement of the movablemicro mirrors 10, 11, 12, 13, 14, 15, 16 due to various factors. Saiddeflection randomness of the movable micro mirrors 10, 11, 12, 13, 14,15, 16 may be compensated for. Moreover, the thickness of the movablemicro mirrors 10, 11, 12, 13, 14, 15, 16 and/or the thickness of anoptional reflective coating of the micro mirrors may vary from one pixelto another, which in turn may affect the reflectivity of the movablemicro mirrors 10, 11, 12, 13, 14, 15, 16. Another difference between theindividual micro mirrors 10, 11, 12, 13, 14, 15, 16 may be that they mayrespond differently to an equivalent potential difference between saidmovable micro mirror 10, 11, 12, 13, 14, 15, 16 and said addresselectrode 410, 411, 412, 413, 414, 415, 416. Given the same potentialdifference between said movable micro mirror 10, 11, 12, 13, 14, 15, 16and said address electrode 410, 411, 412, 413, 414, 415, 416, hingeswith a small cross sectional area will result in a bigger deflectioncompared to hinges with a bigger cross sectional area. Different surfacesmoothness of the micro mirror may also affect the reflectivity as thedistance between the substrate and the micro mirror. Size differences ofthe pixels may also affect the reflectivity.

[0100]FIG. 9 illustrates a side view of the section of the array ofpixels in the Spatial Light Modulator (SLM) as shown in FIG. 1, wheresome pixels are addressed, some are not addressed and all pixels areuncalibrated. Addressed pixels are pixels 11, 12 and 13 and unaddressedpixels are pixels 10, 14, 15 and 16. As can be seen from FIG. 2b, theaddressed pixels 11, 12, 13 are not deflected equally, although theyhave been addressed with the same control signal. This is an example ofthe different response characteristics each mirror may exhibit.

[0101]FIG. 10FIG. 2c illustrates the same side view of the section ofthe array of pixels in the Spatial Light Modulator (SLM) as shown inFIG. 2b, but here with calibrated pixels. As can be seen, the addressedpixels 11, 12 and 13 are deflected equally and the unaddressed pixels10, 14, 15 and 16 are all parallel with the substrate 300. In thealternate case where differences in reflectivity of one pixel comparedto another pixel might exist, the deflection of said pixels would not beequal in order to produce equivalent reflected electromagnetic radiationsignals.

[0102]FIG. 1 shows a Spatial Light Modulator (SLM) 100 comprising a2-dimensional array of pixels, in this embodiment 6 rows with 6 pixelseach, i.e. 36 pixels in total. In reality the SLM may comprise severalmillions of pixels but for reasons of clarity an SLM with few pixels isillustrated in FIG. 1. Pixel 110 is in FIG. 1 printed in black, therebyrepresenting a defective pixel, i.e. said pixel is stuck in a specificposition and does not respond to a calibration. A defective pixel meansa pixel stuck in an on state, an off state or any state between said onstate and said off state.

[0103] In a more general sense, a defective pixel is any pixel theresponse of which is outside acceptable specifications or operatinglimits. If a sensitivity variation to the address signal is determinedto be ±5%, any pixel with a sensitivity diverging more than 5% is adefective pixel.

[0104] Defective pixels will most likely not be controlled in a desiredmanner. In the case of a mirror pixel, said pixel may reflect to less ortoo much of the incident radiation or in the case of LCD pixel saidpixel may be too less transmissive or too much transmissive.

[0105] The pixels in the SLM may be operated in an analog manner. Micromirror pixels are typically operated electrostatically. Piezoelectriccrystals might also operate micro mirrors. By setting the mirrors to afirst potential and by setting an individual address electrodes belowsaid mirrors to a second potential, a difference between said first andsecond potentials will deflect said mirror a certain amount. The biggerthe potential difference between said address electrode and said mirrorelement the more said mirror might deflect. A given potential differencecorresponds to a given deflection for a given mirror and therefore thedeflection can be set to a plurality of states between a nondeflectedstate, i.e., the mirror is electrostatically unatracted, and a fullydeflected state.

[0106]FIG. 2 shows two different writing passes, where a writing stamp100 a belongs to a first writing pass which is written before writingstamps 200 a, 200 b, 200 c and 200 d, which belong to a second writingpass, i.e. writing stamps 200 a, 200 b, 200 c and 200 d are partlyoverlaid on top of writing stamp 100 a. The boundaries between thewriting stamps 200 a, 200 b, 200 c and 200 d are for reason of clarityhighlighted with dashed-dotted lines A-A and B-B. The SLM used may haveone or a plurality of defective pixels but for reasons of clarity onlyone defective pixel 110, 210 a, 210 b, 210 c, 210 d is indicated in FIG.2. An image from the SLM on a work piece will typically cover only asmall portion of the complete pattern, therefore when creating acomplete pattern on a work piece a plurality of different SLM patterns(SLM stamps) are stitched together.

[0107] A specific area in the pattern on the work piece may be writtenwith one or a plurality of writing passes. The writing passes may beseparate physical passes or be exposures of different areas of the sameSLM in a single physical pass. It is also possible to use several SLMssimultaneously where a second pass may be an image from a second SLM. InFIG. 2 two writing passes are used to create the pattern. If one writingpass is used to create the pattern a dose of electromagnetic radiationhigher than the exposure threshold must be used in order to expose aphotosensitive layer (resist layer) arranged on the work piece. If Nwriting passes are used said exposure threshold can be divided N times,i.e. one writing pass is only using a part of the dose required toexpose the photosensitive layer. Every single writing pass may use thesame dose of electromagnetic radiation but said dose may also be dividedunequal between the different writing passes.

[0108] Accordingly, the first and second writing passes might use 75% ofthe threshold dose or any other unequal or equal split of the thresholddose. The defective pixel 110 in the first writing pass can becompensated for by a post compensation pixel 220 belonging to thewriting stamp 200 a in the second writing pass. For reason of claritysaid post compensation pixel 220 is only indicated in stamp 200 d, i.e.,pixel 220 has the same position in stamp 200 a. If the defective pixelis too bright then the post compensation pixel is set to a lower valuein order to compensate the excess of illumination in the first writingpass by said defective pixel 110. In order to further suppress theimpact of a too bright pixel in the first writing pass, a number ofsurrounding pixels 111, 112, 113, 114, 115, 116, 117, 118 (see FIG. 4)to said defective pixel 110 can be set to a lower value, i.e. be set toreflect less electromagnetic radiation. Said surrounding pixels can beused immediately in the first writing pass and/or in later writingpasses.

[0109] In a multiple writing pass scheme as shown in FIG. 2 thedefective pixel will not only affect the first writing pass but also thesecond writing pass. In the four writing stamps 200 a, 200 b, 200 c and200 d in the second writing pass as indicated in FIG. 2 said defectivepixel will show up in four new locations 210 a, 210 b, 210 c, 210 d.

[0110] Because it is established which pixels are defective and it isknown how many pixels the image of the SLM is to be moved in eachdirection, a pre compensation for a impact of the defective pixel in thesecond writing pass may be performed already in the first writing pass.The image of the SLM may for instance be moved, as illustrated in FIG.2, in a direction parallel to both a row and column of pixel, i.e. in anessentially diagonal direction. However, the image of the SLM may onlybe moved along a direction parallel to a column of pixels or along adirection parallel to a row of pixels. Said movement of the image of theSLM may be performed in steps of whole SLM pixels and/or parts thereof.

[0111] Defective pixel 210 d, in FIG. 2, belonging to the writing stamp200 d in the second writing pass, may cause some problem at a placewhere the fully functional pixel 120 belonging to the writing stamp 100a in the first writing pass is located. Therefore a pre compensation maybe performed in the first writing pass in pixel 120 for the defectivepixel 210 d belonging to writing stamp 200 d in the second writing passin the same manner as described above, i.e. with said pixel 120 aloneand/or with adjacent pixels to said pixel 120.

[0112] Having two or more writing passes the pre compensation fordefective pixels in at least one subsequent writing pass may beperformed in at least one prior writing pass. Post compensation for badpixels in at least one prior writing pass may be performed in at leastone subsequent writing pass. I other words, a known bad pixel will makea defective local print in at least one subsequent writing pass which iscompensated for in at least one previous pass and those defective localprints made in at least one previous writing pass may be compensated forin at least one subsequent writing pass.

[0113] In one embodiment of the invention the imaged pattern on the workpiece is written in a single writing pass and defective pixels arecompensated for by at least one of the adjacent pixels to sad defectivepixels. For example in FIG. 4 defective pixel 110 is only compensatedfor by one or a plurality of pixels 111, 112, 113, 114, 115, 116, 117,118.

[0114] In a multipass writing strategy the image from the SLM may bedisplaced N pixel lengths along a row of pixels, along a column ofpixels or along both a row and a column of pixels between at least twoof said writing passes. The image from the SLM may be displaced bymoving a stage on which a substrate to be written is arranged. Betweenone or a plurality of said writing passes the SLM may be displaced onlya fraction of a pixel length in a direction parallel to a row of pixels,in a direction parallel to a column of pixels or along both a row and acolumn of pixels.

[0115] In one embodiment of the calibration process for locating anddetermining which pixel(s) are defective in the SLM comprises thefollowing actions.

[0116] Optionally, the method begins with a calibration of the dose ofelectromagnetic radiation. A CCD camera has a specific working range ofelectromagnetic radiation doses. Preferably, the dose of electromagneticradiation lies around 0.8*max range of the CCD camera. With a too lowdose projected onto the CCD, the signal to noise ratio will in somecases be unacceptable low. With a too high dose projected onto the CCDcamera, the CCD camera will be over saturated, with the result of aninaccurate measurement.

[0117] The calibration of the dose of electromagnetic radiation may beperformed by starting with an SLM with all pixels unaddressed, i.e. nocontrol signals applied to the pixels. The electromagnetic radiation isprojected on said CCD via said SLM. The dose of electromagneticradiation is measured on said CCD. After said measurement, the dose maybe corrected by adjusting the electromagnetic radiation source.Increasing or decreasing the power of said pulsed electromagneticradiation source might perform said dose adjustment. A higher power willresult in a higher dose on said CCD and a lower power will result in alower dose on said CCD.

[0118] Optionally, said imaging detector, e.g., the CCD camera, isprecalibrated. Said precalibration of the CCD camera may be performed byprojecting a known electromagnetic radiation beam with approximately thesame wavelength, e.g. a discharge lamp, and an interference filter toselect a narrow wavelength range close to the wavelength of the exposurebeam, and measuring the dose in each pixel of the CCD camera. The objectof this precalibration is to be sure about that the same doseilluminated at each pixel in the CCD camera will be measured as the samedose, i.e. every pixel after said precalibration shall be equallysensitive to electromagnetic radiation, and by so doing the accuracy ofthe measurement will be enhanced.

[0119] The calibration of the SLM continues with finding the SLM averageintensities as a function of control signal. The object of finding theSLM average intensities as a function of applied control signal is tofind a control signal for a predetermined dose, for example zero doses.This is performed by looping the control signals, for example from0-255, for each pixel. For a given control signal, all pixels aremeasured and the average for the measured pixels is calculated. Anexample of a number of pixel intensities as a function of appliedcontrol signal is shown in FIG. 12. In said figure the horizontal linesA and B represent the dynamic range, representing levels attainable byall mirrors.

[0120] When searching for the control signals for the next dose signalone can guess the derivative of the calibration curve in order to reducethe number of steps to find the right control signal for each pixel. Asthe number of known points on the calibration curve increases the stepsto find the right control signal for a specific dose value decreasessince the information about the calibration curves are enhanced by saidknown points.

[0121] Next the pixels in the SLM are mapped with the pixels in the CCD.The object of this is to establish a known relation between the pixelsin the SLM and the pixels in the CCD camera. First a coarse grid ofclusters of pixels in said SLM is measured in the CCD camera. Forexample, a 5 by 5 array cluster with 30 pixels in between each cluster.This will produce a distinctive signal on the CCD. The SLM is providedwith a special pattern in order to be able to know which part of the SLMthat is studied. When only a part of the SLM area is studied at a time,it is important to know which part that is studied. The cluster ofpixels may be moved from one part to another in the SLM area. The pixelsin the cluster are set to a value, which is distinctive from the nearbyunaddressed pixels.

[0122] The image on the CCD at this stage may correct for translationdeviations, scale errors, mirror effects and rotational errors betweensaid SLM and said CCD according to the formula {overscore(A)}=M*S*R*({overscore (E)}−t), where {overscore (A)} is the CCDcoordinate, M=mirroring, S=scale factor, R=rotation, {overscore (E)}=SLMcoordinate and t=translation. {overscore (E)} and {overscore (A)} arevectors containing coordinates for the SLM pixel and CCD-pixelrespectively. M may for instance be the 2×2 unity matrix or mirroringmatrix. S may be any figure between 0-infinity, but preferably between0-3. R may be a 2×2 matrix with cos(α) in the upper left position,−sin(α) in the upper right position, sin(α) in the lower left positionand cos(α) in the lower right position, where α is typically a fewmilliradians. In a general case the mapping is a non linear mapping,having for instance a t factor that is a function of the coordinates.

[0123] In a preferred embodiment of this invention the single pixels inthe SLM are not resolved in the CCD camera.

[0124] Secondly a finer grid of the cluster of pixels in said SLM ismeasured on the CCD. At this stage, with a finer grid, the number ofpixels at each pixel cluster in the SLM may be a 3 by 3 array with forexample 20 pixels in between each cluster.

[0125] Thereafter a further refinement of the cluster of pixels in theSLM is measured on the CCD, for example with a single pixel in thecluster with 10 pixels in between.

[0126] As a further refinement of the mapping of SLM to the CCD anon-linear correction may be added. This means that y=MSR(x−t)+nonlinearcorrection. This non-linear correction is for example computed byassigning second order polynomial expressions with unknown coefficientsa-j. Such polynomial expression may be:

nc _(—)1=ax+by+cx ² +dy ² +exy, nc _(—)2=fx+gy+hx ² +iy ² +jxy,

[0127] where nc_(—)1 is the nonlinear correction for coordinate x andnc_(—)2 is the nonlinear correction for coordinate y. If the correctionvaries with (x,y), as in this case, a position dependent non-linearcorrection U can be fitted to the function (nc_(—)1, nc_(—)2)(x,y) byusing the least square fit method.

[0128] In a further improvement, a correction for the spot positionrelative to a CCD pixel grid may be applied to remove or reduce Moireeffects, due to an insensitive area between the CCD pixels or similareffects. The magnification in the projection system may or may not beadjusted, so that the image of the sub-matrix is adjusted with the pixelpattern on the CCD, e.g. the CCD may have one pixel per two pixels inthe SLM or another rational relation. The CCD pixels typically have acapacity of 100.000 electrons. In the measurement region formed byseveral pixels the capacity may be larger by a numerical factorrepresenting the number of pixels, e.g. 4 or 16 as shown in FIG. 5. Thetypical number of electrons in a region is 200.000 and the number has astatistical distribution (Poisson distribution). To average this randomeffect as well as other randomness every measurement is repeated Ntimes. At the same time Moire effects can be averaged if the image ismoved over the CCD-camera during the N measurements.

[0129] The CCD-camera is for example a camera from Kodak® KAF 1600 withapproximately 1000*1600 pixels and sensitivity for the wavelength used,e.g. 248 nm or 197 nm. Typically this involves converting the radiationto visible light by a fluorescent dye, but camera chips which aredirectly sensitive to short wavelength, e.g. 248 nm are also available.

[0130] In the next step the control signal is searched for, which givesrise to a predetermined dose value on the CCD. By having a goodknowledge of where a specific SLM pixel will be detected on the CCD,said image on the CCD camera can be corrected in order to arrive at saidpredetermined value for all pixels. FIG. 11 illustrates a typicalresponse on the CCD camera for a projected electromagnetic radiationonto an uncalibrated SLM. Vertical lines 25 represent a boundary betweentwo pixels. As can be seen some mirrors are giving rise to too muchreflectance and some mirrors are giving rise to too low reflectance thandesired. As we know the relationship between the SLM and CCD pixels wecan change the state of the SLM mirrors/pixels, which are too reflectiveor too less reflective. By changing the state of those mirrors/pixelsand projecting a new image on the CCD a new response, will appear. Thechange of state of the mirrors, are changed in finer steps than thedifference between the predetermined value and the factual value. Thisis done in order to be sure to have a method that is convergent. Afterhaving changed the pixels and projected the image of the SLM onto theCCD a number of times the pixels are calibrated. A condition forstopping the calibration for this particular predetermined dose value,might be that the standard deviation of the detected mirrors on the CCDless than 0.5%.

[0131] Next the different predetermined dose values are stepped throughin the same way as described above. After this, a good knowledge aboutthe reflectance as a function of voltage for each mirror exists.

[0132] Optionally the common maximum and minimum reflectance attainableby all mirrors are to be found. Between those values, the inverse, i.e.voltage as a function of reflectance, is well defined for all mirrors,and those are the functions that is to be find approximate expressionsfor, using a limited space of storage for each mirror. In FIG. 12,reflection as a function of voltage is shown for a number of differentmirrors. Since the entire array has to have common “white” and “black”levels (as seen by the CCD), the dynamic reflectance range for the wholearray will be limited by levels attainable by all mirrors (as indicatedby lines A and B in FIG. 12. Depending on the requirement on the dynamicrange, we may have to exclude the use of some mirrors beyond those beingdefective. Such mirrors may still be used, although with a largercompensation error in “black” or “white” levels. When the “white” and“black” levels are selected, we may begin to calibrate each individualmirror within that reflectance range.

[0133] A way of finding the common expression for the voltage as afunction of reflectance doze is by interpolation using Fourier methods.For instance each mirror is calibrated using four parameters. FIG. 13illustrates the voltage as a function reflectance doze. In this figurethe common “black” level 305 and the common “white” level 310 isindicated with vertical lines. The first two calibration parameters canbe identified as the driving voltage at the intersections 315, 320 ofthe mirror response and the “black” and “white” levels 305, 310. Theremaining calibration parameters are obtained by calculating the Fouriercoefficients of the difference between the mirror voltage and thestraight line 325 that interpolates the reflectance between the “black”and “white” levels. Since we have, by construction, zero error at theendpoints, it is sufficient to use sin(πx) and sin(2πx) as the harmonicsfunctions in a Fourier expansion. The variables x equals(z-z_“black”)/(z_“white”-z_“black”), which has the closed range x=(0,1).

[0134] If assigning two calibration parameters to describe the straightline 325, one may use another two as coefficients for the base functionssin(πx) and sin(2πx). A calibration expression would then bez=a+bx+c(sin(πx))+d(sin(2πx)), where a, b, c and d are unique for everypixel and sin πx and sin 2πx are common to all pixels.

[0135] Alternatively, instead of expanding the difference of thestraight line that interpolates the reflectance function (see FIG. 8)into just sin(πx) and sin(2πx), the difference may be expanded at alarger number of Fourier components. Having M mirrors (and consequentlyM functions) and expanding into N components gives us a matrix A havinga dimension NXM. The base functions may now be chosen by selecting thetwo eigenvectors of (the square matrix) AA^(t), where t stands fortranspose, with the largest eigenvalues. The base functions obtained inthis way are still sine like (although the choice of Fourier base isinsignificant), but fit the data without average (or systematical)errors.

[0136] The calibration coefficients a, b, c and d are found by solvingAc=Y, where A is a 4×4 matrix and Y is a 4×1 vector. The elements of thematrix are${A_{ij} = {{\sum\limits_{m}{{w( x_{m} )}{f_{i}( x_{m} )}{f_{j}( x_{m} )}\quad {and}\quad {Yi}}} = {\sum\limits_{m}{{w( x_{m} )}y_{m}{f_{i}( x_{m} )}}}}},$

[0137] where Y is the voltage at some (normalized) reflectance samplex_(m) and w(x) is the weight function which can be chosen to unity. Thetwo functions f1 and f2 are the constant function and the linearfunction f(x)=x. The remaining two that were used, are those derivedfrom a sinc(x)-function. If the weight function, w(x) is chosen to unityone will obtain calibration coefficients (c) that minimize the variance.If one also choose two of the base functions to be sin(πx) and sin(2πx)one will obtain solutions very similar to the Fourier expansion. Thedifference between these two originates only from the requirement thatthe constant and the linear functions are used to interpolatecalibration data (at the end points) exactly in the Fourier case, whilethey are chosen freely by the least square algorithm. Consequently, theleast square fit produces the smallest average error, but is notguarantied to be exact at the endpoints.

[0138] The algorithm for the compensation is:

U(x)=c ₁ +c ₂ x+c ₃ f ₃(x)+c ₄ f ₄(x)

[0139] In another alternative embodiment of the invention a beam ofelectromagnetic radiation is projected on to at least a part of the SLM.The radiation source may for example be a laser, which may be continuousor pulsed. The part of the SLM, on which said electromagnetic radiationis projected, is used for said calibration. The part may for instance beone tenth of the SLM area, the half area of the SLM or the complete areaof the SLM.

[0140] Next an image on an imaging detector is formed of said part ofthe SLM. Said imaging detector may for example be a CCD-camera,MOS-camera or a charge injection device. In said image darker andbrighter regions may appear due to the different deflection states andreflectivity of the pixels in the SLM. The image corresponds to theimage on the work piece 60.

[0141] Thereafter at least two pixels out of said part of the SLM aredriven to a sequence of applied pixel control signals while measuringthe dose from said individual pixels on said imaging detector. Said atleast two pixels may for instance be a sub-matrix, where pixels in thesub-matrix are separated by pixels in a non-addressed state, i.e. noapplied pixel control signal applied to the pixels.

[0142] Finally pixel calibrating data is computed from the measured dosedata as a function of applied pixel control signal.

[0143] Optionally said imaging detector, e.g. the CCD camera, isprecalibrated. The CCD-camera may be precalibrated by projecting a knownelectromagnetic radiation beam with approximately the same wavelength,e.g. a discharge lamp and an interference filter to select a narrowwavelength range close to the wavelength of the exposure beam, andmeasuring the dose in each pixel of the CCD-camera. The object of thisprecalibration is to be sure about that the same dose illuminated ateach pixel in the CCD camera will be measured as the same dose, i.e.every pixel after said precalibration shall be equally sensitive toelectromagnetic radiation, and by so doing the accuracy of themeasurement will be enhanced.

[0144] The sub-matrix comprising at least two pixels is chosen in ordernot having to measure pixel by pixel in said SLM. The selected pixels inthe SLM are not located next to each other but having a number of pixelsin between, in order to reduce the likelihood of treating one spot onthe CCD camera as coming from two or more pixels in the SLM, i.e.distinctive spots from individual SLM pixels on said CCD camera. Thedistance between two pixels in said sub-matrix is 5 pixels in eachdirection, but other separation distances may be used. As a generalrule, a value 6σ of the dose of the radiation from the pixels in saidsub-matrix may be a measure of the distance between the pixels in saidsub-matrix.

[0145]FIG. 5 shows schematically a top view of the relation between thepixels in an imaging detector 250 and an energy distribution 275 fromindividual pixels in a SLM. The energy from the SLM pixels may take theform of a Gaussian distribution. In FIG. 5 the Gaussian distribution isschematically represented by circles, where very close circles, as inthe center of each Gaussian distribution, represent high energy, andwidely separated circles represent lower energy. As also can be seenfrom the same FIG. 5, is that the separation of the Gaussiandistributions on the imaging detector 250 in a X direction is widercompared to the separation of the same distributions in a Y direction.In FIG. 5 the distance between the center of the Gaussian distributionsin the X direction is 5 imaging detector pixels, while in the Ydirection the distance between the same Gaussian distributions is 4imaging detector pixels.

[0146] Optionally the pixels in the SLM are mapped with the pixels inthe CCD camera to establish a known relation between the pixels in theSLM and the pixels in said CCD-camera. In this mapping step the centerof the dose of the radiation from the pixels in the SLM can be alignedessentially in the center of the pixels in the CCD camera. Alignmenterrors of the order of 0.5 pixels cause the calibration algorithm tocreate spurious patterns in the image. This may be performed bymeasuring a center of the spot on the CCD and the position of the SLMimage on the CCD is adjusted by a translation, magnification and/orrotation to fit the pixels on the CCD as described above in connectionwith the previous embodiment.

[0147] Alternatively the position of the spots on the CCD is measuredand a region is computed for each pixel, so that a computer can assignthe image at a particular location on the CCD to a corresponding SLMpixel.

[0148] Alternatively first a coarse grid of very few pixels in said SLMis measured on said CCD camera, for example a number of pixels, e.g. acluster of 5 times 5 pixels may be chosen to begin with so a distinctivesignal is given on the CCD. Said cluster of 5 times 5 pixels may bemoved from one corner to another in the rectangular SLM area. The pixelsin the cluster are set to a value, which is distinctive from the nearbyunaddressed pixels.

[0149] The image on the CCD at this stage may correct for translationdeviations between sad SLM and said CCD, i.e. scale errors, rotationalerrors etc. In the preferred embodiment of this invention the singlepixels in the SLM are not resolved in the CCD camera.

[0150] Secondly a finer grid of the cluster of pixels in said SLM ismeasured on the CCD in order to locate which pixels in the SLM willcreate dose values on the CCD and where. At this stage with a finer gridthe number of pixels at each pixel cluster in the SLM is reduced to 3times 3 pixels with for example 10 pixels in between each cluster.

[0151] Thereafter a further refinement of the grid of cluster of pixelsin the SLM is measured on the CCD, this time for example with singlepixels in the SLM with 5 pixels in between such.

[0152] As a further improvement, a correction for the spot positionrelative to a CCD pixel grid may be applied to remove or reduce Moireeffects due to insensitive area between the CCD pixels or similareffects. The magnification in the projection system may or may not beadjusted, so that the image of the sub-matrix is adjusted with the pixelpattern on the CCD, e.g. the CCD may have one pixel per two pixels inthe SLM or another rational relation. The CCD pixels typically have acapacity of 100.000 electrons. In the measurement region formed byseveral pixels the capacity may be larger by a numerical factorrepresenting the number of pixels, e.g. 4 or 16 as shown in FIG. 5. Thetypical number of electrons in a region is 200.000 and the number has astatistical distribution (Poisson distribution). To average this randomeffect as well as other randomness every measurement is repeated Ntimes. At the same time Moiré effects can be averaged if the image ismoved over the CCD-camera during the N measurements.

[0153] The CCD-camera is for example a camera from Kodak® KAF 1600 withapproximately 1000*1600 pixels and sensitivity for the wavelength used,e.g. 248 nm or 197 nm. Typically this involves converting the radiationto visible light by a fluorescent dye, but camera chips which aredirectly sensitive to short wavelength, e.g. 248 nm are also available.

[0154] In order to calibrate all pixels in the illuminated part of theSLM said at least two pixels at a time are changed and driven to asequence of applied pixel control signals. Now we have knowledge aboutthe doze on the CCD as a function of control signal for each pixel. Withthe knowledge of doze as a function of applied voltage for every pixel astate is computed representing average zero dose of electromagneticradiation onto the detector arrangement out of the measured dose data.

[0155] Thereafter at least the most adjacent pixels to the pixels insaid sub-matrix are arranged in said computed state.

[0156] As can be seen from FIG. 8, the pixels 10, 11, 12, 13, 14, 15, 16in an electrostatically unattracted state may be in a random state ofdeflection and therefore may contribute to the radiation dose at aspecific CCD-camera pixel and thereby decrease the accuracy of themeasured dose. In order to eliminate or at least reduce an inaccuracy inthe measured dose coming from a specific SLM pixel, at least the closestSLM pixels to said at least two pixels in said part of the SLM arearranged in said computed state.

[0157] Not only the most adjacent pixels to said at least two pixels insaid SLM may be set to said computed state, preferably all pixels otherthan said at least two pixels in said SLM are set to said computedstate.

[0158] Thereafter at least two pixels at a time in said SLM are drivenagain to a sequence of applied pixel control signals while measuring thedose of the electromagnetic radiation, while at least the most adjacentpixels to said at least two pixels in the SLM are set to said computedstate. After having completed the calibration of all pixels in said partof the SLM a second time, a new state is computed for each pixelcorresponding to average zero dose of electromagnetic radiation onto theimaging detector out of the second measured dose data. This procedure isrepeated until for example the standard deviation of the measuredintensities is below 0.5%.

[0159] The dose of the pixels in said sub-matrix is measured in said CCDcamera. The CCD camera does not have to resolve single pixels, becauseonly one sub-matrix is changed at a time. The change of a single pixelcan be inferred from the measurement. The density of the sub-matrix maybe chosen to make the spots on the CCD essentially non-overlapping. TheCCD may or may not have the same number of pixels as the SLM. The lightin the CCD-camera image within a certain area is assumed to come fromone pixel in the SLM, provided that the surrounding SLM pixels do notchange.

[0160] Optionally a compensation for energy variations in differentelectromagnetic radiation pulses is performed. The calibration of thepixels may be performed by illuminating said sub-matrix of pixels insaid SLM by a pulsed laser and measuring and calculating the dose fromone or a plurality of laser pulses and correcting the measured CCD datafor the measured pulse energy.

[0161] Said projecting of electromagnetic radiation from said sub-matrixof pixels onto the detector arrangement for measuring dose ofelectromagnetic radiating may be performed after Fourier filtering ofsaid electromagnetic radiation. In FIG. 3 a beam splitter 90 is arrangedbetween the spatial filter 70 and the first lens arrangement 50.

[0162] After having measured the dose for the pixels in said sub-matrixfor a given voltage applied to said pixels, said voltage applied to thepixels in said sub-matrix is changed and the procedure is repeated for anumber of different voltages. For example may the dose from a maximumvalue to a minimum value be divided into 65 values. After having appliedall different voltages to the sub-matrix of pixels the procedure may berepeated for all sub-matrices 200 in order to cover said part of the SLMonto which the beam of electromagnetic radiation is projected. Thesub-matrix may or may not change pattern from one position to another insaid 2-dimensional array of pixels.

[0163] The beam of radiation is projected on to the other parts of theSLM in order to calibrate all pixels in said SLM. Preferably the samesize of the beam is used but the size may change with the result ofcovering different sizes of the parts of the SLM.

[0164] The pixel correction data is generated either by storing everydose value for a given control signal, which in this case is a potentialdifference between the mirror and the address/control electrode, forevery pixel in a data base or more preferably by transforming themeasured dose as a function of applied voltage onto said pixel as atransfer function. Said transfer function is preferably a given formulae.g. C1+XC2+C3T3(X)+C4T4(X) equal for every pixel. The procedure forcomputing the constants C1, C2, C3 and C4 may be similar to what isdescribed in connection with the previous embodiment. The procedure forfinding the base functions may also be similar to what is described inconnection with the previous embodiment. For example C1+xC2 is theformula of the straight line T3(x) and T4(x) may in this case twotabulated functions. T3 and T4 can be chosen so that the formula givesan adequate description of all pixels.

[0165]FIG. 12 shows schematically a flow chart of another embodiment ofthe method according to the invention for calibrating pixels in aSpatial Light Modulator (SLM).

[0166] In FIG. 6 it is shown a typical example of a relation betweenmeasured dose on the imaging detector and applied pixel control signalfor a pixel in a SLM, this is illustrated by curve 275. In the same FIG.6 a curve 260 represents an amplitude of the electromagnetical field asa function of applied pixel control signal. The relation between thedose curve 275 and the amplitude curve 260 is that the dose curve 275 isthe square of the amplitude curve 260.

[0167] The dose curve can for example be approximated with a (sinx/x)²function and the amplitude curve would then be a sinx/x function.

[0168] Another way of finding as correct zero dose from a single pixelas possible is to use the above-mentioned fact that the dose curve canbe approximated with a (sinx/x)² function. If the local maximum at 275in measured the local minimum points can easily be computed out of saidfunction. The reason why local maximum is easier to measure than thefactual minimum point, is that the signal in the CCD-camera at theminimum point will disappear in the noise that is always present this isnot the case with the local maximum at point at the dose curve.

[0169] The image rendering engine of the present invention can be usedin conjunction with a fracturing engine, rasterizing engine and drivecircuit. FIG. 14 provides a data path overview. This data path beginswith preprocessed geometry data 1201 as input. Preprocessed geometrydata may be the output of a computer-aided design system. Preprocessingmay reduce hierarchical or iterative information and effectively flattenthe geometry representation stream. Data fetching 1202 typicallyincludes obtaining preprocessed geometry data from a secondary storagedevice. Geometry conversion 1203 is the process in which geometries areconverted to renderable fixed-point geometries (RFG). Fracturing 203 isthe process of partitioning geometries into different windows and subwindows which correspond, in a micromirror implementation, to stamps andrendering windows of the stamp. The output of the fracturing engine isgeometry data in one or more specified record formats. The recordsrepresent geometric figures, such as polygons and groups of polygons. Itis useful to represent the fractured data as trapezoids, where trianglesand rectangles are sub classes of trapezoids. One of the parallel edgesof the trapezoid may have a zero or near-zero length, to represent atriangle. Another useful representation of fractured data is astriangles or chains of triangles. Most aspects of the present inventionare equally suited to trapezoids, rectangles, triangles or otherpolygons or geometric figures. Coordinates of the polygon comers may begiven with a sub-pixel or half sub-pixel resolution of 7 or bits ormore, to support an accuracy of one 64^(th) or 128^(th) of a pixel orgreater. Higher and lower bit resolutions may be used, depending on thedesired accuracy and the characteristics of the image projectiontechnology.

[0170] The image-rendering engine 210 includes a variety of components.Expansion 1211 is the process of expanding geometry iteration prior torendering. Fractured geometry may be received as iterated RFGs, withrepeated geometric figures or repeated groups of geometric figures.Expansion ungroups the RFGs so they can be processed individually.Rendering 1212 is the process of converting polygons, includingrenderable fixed-point geometries, to rasterized images. The renderingprocess is carried out on multiple rendering processors. Super sampling1212 is the process of sampling the micropixel resolution image andcalculating grayscale pixel values. Alternative weighting schemes forsuper sampling are discussed below. Edge displacement 1213 is theprocess of shrinking or expanding geometries, for instance to compensatefor proximate and stray radiation by laser proximity correction (LPC) orby optical proximity correction (OPC). Image correction 1214 is theprocess of compensating for non-linearities and minor defects in theoptical path, the placement of the stage or another feature of theprojection system. This may include non-linear image recoupling.Illumination conversion 1215 takes into account factors such as overlapbetween projected regions, variations in exposing radiation, andmulti-pass writing. Mirror compensation 1216 applies pre-calibratedfactors to compensate for idiosyncrasies of individual mirrors, when theprojection system uses a micromirror array. Mirror compensation factorscan be used to compensate for differential response to voltages, forchange in response during the course of a work cycle, for a dead pixelin an array, or similar characteristics of a micromirror array.Additional components can be added to the rendering engine 1210 asneeded and as appropriate to the projection system being used.

[0171] The drive circuit 1220 includes composition 1221 and modulation1222 processes. Composition 1221 is the process of combining resultsfrom several rendering processes into one or more data streams to whichmodulation is responsive. Use of a composer allows the number ofrendering modules 1330 to be scaled. For instance, the number ofrendering modules may be increased from 10 to 12 by modification ofcomposer parameters, without changing the interface to the modulationsystem. In one type of micromirror system, one data stream may be usedfor modulation, to set individual micromirrors before flashing themicromirror array with radiation. In another type of micromirror system,the number of data streams may match the number of micromirrors or afactor of the number of micromirrors, if the micromirrors are used forscanning a work piece. In a conventional scanning system, the number ofdata streams may match the number of scanning beams used. Modulation1222 is the process that converts concentrated data into driving valuesfor the projection system. For a micromirror system, a digital-to-analogconverter can be used to produce analog voltages that are applied toindividual mirror elements. For a scanning system, drive signals may beused to control an acousto-optical modulator that modulates theradiation beams or an equivalent control element for electron, ion orparticle radiation.

[0172] A non-linear transform may require application of a pixelresampling gradient to each pixel being resampled. Alternatively,gradients for each pixel could be sampled by a convolution kernel toproduce an output pixel value. The neighbourhood of the convolutionkernel will depend on the maximum allowed magnitude of the gradient. Aone pixel gradient could be sampled by a 3×3 kernel; a two pixelgradient by a 5×5 kernel.

[0173] A projection system typically also includes a sweep 1230 and areticle 1240. The sweep 1230 carries image information across the fieldof the reticle 1240, which is being exposed to radiation. The reticle1240 is the work piece against which the projection system operates.

[0174]FIG. 15 illustrates a flow chart of a further embodiment accordingto the invention. First the geometries are converted into steeringsignals for each pixel according, for instance, to what was described inconnection to FIG. 14. Thereafter it is determined where bad pixels arelocated and determined if said bad pixels are located in a criticalposition. A bad pixel, for example a black pixel, located near an edgemay introduce an error in the pattern.

[0175] A bad pixel, i.e., a mirror stuck to a limited range of angles,may be identified in a variety of ways. It is possible to calibrate theSLM and find the defective pixels by searching for elements that haveunusual calibration properties. However, the calibration procedure maybe negatively influenced by the bad pixels.

[0176] Generally, with a non-calibrated SLM said SLM may be addressedwith steering signals, for instance voltages, and an image of the SLM isrecorded with a digital camera. The image is then processed using imageanalysis. One or more images (at different voltages) may be required toidentify position and magnitude of the bad pixel(s).

[0177] In one embodiment are all mirrors driven with a constant voltage.Said voltage may represent any grayscale value. The image is recordedwith a CCD camera and image analysis is used to detect the bad pixel.For example may a gradient field be computed from the recordedCCD-image. A divergence of the gradient (Del2) is computed, where Del2is the Laplace's differential operator, in order to identify positionsin the image where a spatial second derivative of the intensity hasextremes. Said extremes, for example a maximum and minimum, mayrepresent bad pixels. The Del2 function may be set with a thresholdvalue, and any position above said threshold value may represent a badpixel. The procedure may be repeated with the mirrors driven to at leastone other constant voltage, preferably the constant voltages are wellseparated from each other. Suspected bad pixels are considered to beconfirmed if the same pixel shows up on repeated measurements or withcomplementary measurements.

[0178] Having images representing different settings for the pixels adifference between said image may be calculated. The difference has alocal minimum at a stuck or damaged pixel.

[0179] For a binary (on-off) SLM, it may in some configurations bedifficult to use the before mentioned methods. Instead a pattern isapplied on the SLM. Bad pixels have largest influence at feature edges.A pattern containing parallel black and white lines or a chessboardpattern will highlight the pixels that are on the edges. Said patternare preferably near the optical resolution limit. By driving a sequenceof patterns, e.g., parallel lines, to the SLM and taking CCD images ofeach pattern will allow identification of defective elements although itmay not be optically resolved in the images. The lines in the differentimages are moved for identification where a defective pixel is located.The CCD images are compared with pattern data. For example, the SLM maybe addressed with lines and spaces and its complement in two images. Byanalyzing the linewidth in the two images a bad pixel can be detectedand its magnitude determined. For example, a bad pixel stuck to a whitevalue gives a linewidth error only when it is on black area and viceversa. However, a gray pixel give errors both in a black and a whitearea. The difference of the errors contains information about themagnitude of the defect.

[0180] To compensate for a black pixel the neighbouring pixels may beset to a more intense state, i.e., a state in which more electromagneticradiation will be reflected onto the workpiece. If bad pixel is near anedge, software may adjust the neighboring pixels. In practice, this canbe accomplished by changing the pixel value, i.e., the degree ofdeflection of the adjacent mirrors, or changing the transfer function ofthe adjacent pixels. This type of compensation may be used in one or aplurality of writing passes. The change of the neighbouring pixels maybe performed by on line calculation or using a lock-up table.

[0181] The invention allows the correction for bad pixels that reducesthe error dramatically in a multipass scheme with constant correctionsto the transfer functions. With only one or two passes the correctionmay not be sufficient to satisfy the requirements posed in a particularcase. In a more elaborate embodiment the correction is made dependent onthe actual pattern. Typically only certain geometrical cases arecritical and those can be characterized and suitable bad pixelcorrection data and can be stored in a look up table or in algorithmicform. During printing, the pattern near the bad pixels is analysed inreal time, the suitable correction, if necessary, is identified andapplied. With only a few defective pixels per SLM the computing work ismanageable. In this way an almost perfect correction can be made withanalog mirrors and a much improved correction can be accomplished withbinary (on-off) elements.

[0182] In one embodiment, bad pixels at a distance of one pixel insideor outside the feature edge is regarded as bad pixels in criticalposition. In another embodiment bad pixels at a distance of two pixelsinside or outside the feature edge is regarded as bad pixels in criticalposition. In still another embodiment bad pixels at a distance of threepixels inside or outside the feature edge is regarded as bad pixels incritical position.

[0183] In yet another embodiment of the present invention a plurality ofSLMs are used for compensation of bad pixels. In FIG. 16 an exemplaryembodiment of a setup is illustrated. Said figure comprising a first SLM1610, a second SLM 1620 and a beamsplitter 1630. Said arrangement isintroduced instead of SLM 30 in FIG. 3. The individual SLMs, for examplefirst SLM 1610 and the second SLM 1620, may be fed with the same patterndata. The calibrating functions for individual pixels in said first andsaid second SLM are however unique. The SLM area may be calibratedtogether so that each pixel in the first SLM corresponds to a group ofpixels in the second SLM. This may be done in both ways. By so doing onewill get a one to four relationship of pixels in both directions.Differences of the SLM will in this way be corrected for. Somegeometrical systematical errors in the SLM itself may be cancelled outif the first SLM is rotated 180 degrees relative to the second SLM. Thefirst and second SLM may be illuminated by the same intensity ofelectromagnetic radiation. However, by illuminating said first and saidsecond SLM by different intensities a further degree of grayscaling maybe accomplished. The number of grayscales will in such embodiment dependupon a relation and absolute value of the two intensities. For example,if the pixels in said first and second SLM may be set to 16 levels andradiation from said second SLM is {fraction (1/16)} of the radiationfrom said first SLM the number of grayscale levels is 16×16=256 levels.Speckle may also be reduced or cancelled by using a plurality of SLMs.

[0184] Thus, although there has been disclosed to this point particularembodiments of the apparatus for patterning a work piece, it is notintended that such specific references be considered as limitations uponthe scope of this invention except in-so-far as set forth in thefollowing claims. Furthermore, having described the invention inconnection with certain specific embodiments thereof, it is to beunderstood that further modifications may suggest themselves to thoseskilled in the art, it is intended to cover all such modifications asfall within the scope of the appended claims.

1. A method for compensating the impact of at least one defective pixelwith a known position in at least one spatial light modulator (SLM) whencreating a pattern of said at least one SLM on a work piece covered atleast partly with a layer sensitive to electromagnetic radiation,comprising the actions of: providing a source for emittingelectromagnetic radiation, illuminating by said radiation said at leastone SLM having a plurality of modulating elements (pixels), projectingin a writing pass an image of said modulator on said work piece,performing a compensation for defective pixels in at least one otherwriting pass.
 2. The method according to claim 1, wherein saidelectromagnetic radiation is a pulsed laser source.
 3. The methodaccording to claim 1, wherein said compensation for said at least onedefective pixel is performed by using a single compensation pixel foreach defective pixel.
 4. The method according to claim 1, wherein saidcompensation for said at least one defective pixel is performed by usinga plurality of compensation pixels for each defective pixel.
 5. Themethod according to claim 1, wherein said at least one SLM isilluminated by equivalent radiation dose in the different writingpasses.
 6. The method according to claim 1, wherein said at least oneSLM is illuminated by different radiation intensities in the differentwriting passes.
 7. The method according to claim 1, wherein said atleast one SLM is a transmissive Spatial Light Modulator.
 8. The methodaccording to claim 1, wherein said at least one SLM is a reflectiveSpatial Light Modulator.
 9. The method according to claim 1, wherein thepixels in said at least one SLM is operated in an analog manner.
 10. Amethod for compensating the impact of at least one defective pixel witha known position in at least one spatial light modulator (SLM) whencreating a pattern of said at least one SLM on a work piece covered atleast partly with a layer sensitive to electromagnetic radiation,comprising the actions of: providing a source to emit electromagneticradiation, illuminating by said radiation said at least one SLM having aplurality of modulating elements (pixels), projecting an image of saidat least one SLM on a detector arrangement to measure a dose ofradiation performing a compensation of said defective pixel by at leastone of the most adjacent pixels in said at least one SLM.
 11. The methodaccording to claim 10, wherein said compensation is performed byassigning each of said at least one of the most adjacent pixels by avalue given by subtraction of an intended pixel value by a actual pixelvalue, divided by the number of most adjacent pixels used forcompensation.
 12. A method for compensating the impact of at least onedefective pixel in at least one spatial light modulator (SLM) whencreating a pattern of said at least one SLM on a work piece covered atleast partially with a layer sensitive to electromagnetic radiation,comprising the action: setting the pixels in said at least one SLM in apredetermined state, illuminating by a radiation source said at leastone SLM, projecting the image of said at least one SLM on a measuringdevice, measuring the dose of the pixels, identifying defective pixels,performing a compensation for said defective pixels in at least onewriting pass by means of pixels in said at least one SLM other than saiddefective pixels.
 13. A method for compensating the impact of at leastone defective pixel with a known position in at least one spatial lightmodulator (SLM) when creating a pattern of said at least one SLM on awork piece covered at least partly with a layer sensitive toelectromagnetic radiation, comprising the actions of: providing a sourcefor emitting electromagnetic radiation, illuminating by said radiationsaid at least one SLM having a plurality of modulating elements(pixels), projecting in a first writing pass an image of said modulatoron said work piece using a first set of pixels in said at least one SLM,performing a pre compensation of at least one defective pixel in atleast one subsequent writing pass by at least one compensation pixel inat least one prior writing pass, projecting in at least a second writingpass said image of said modulator on said workpiece using at least asecond set of pixels in said at least one SLM.
 14. The method accordingto claim 13, further comprising the action of: performing a postcompensation of at least one defective pixel in at least one priorwriting step by at least one compensation pixel in at least onesubsequent writing pass.
 15. A method for compensating the impact of atleast one defective pixel with a known position in at least one spatiallight modulator (SLM) when creating a pattern of said at least one SLMon a work piece covered at least partly with a layer sensitive toelectromagnetic radiation, comprising the actions of: providing a sourcefor emitting electromagnetic radiation, illuminating by said radiationsaid at least one SLM having a plurality of modulating elements(pixels), projecting in a first writing pass an image of said modulatoron said work piece using a first set of pixels in said at least one SLM,performing a post compensation of at least one defective pixel in atleast one prior writing pass by at least one compensation pixel in atleast one subsequent writing pass, projecting in at least a secondwriting pass said image of said modulator on said workpiece using atleast a second set of pixels in said at least one SLM.
 16. The methodaccording to claim 15, further comprising the action of: performing apre compensation of at least one defective pixel in at least onesubsequent writing step by at least one compensation pixel in at leastone prior writing pass.
 17. The method according to claim 13 or 15,wherein said electromagnetic radiation is a pulsed laser source.
 18. Themethod according to claim 13 or 15, further comprising the action of:including at least one pixel in said first set of pixels in said atleast second set of pixels.
 19. The method according to claim 13 or 15,wherein said compensation for said at least one defective pixel isperformed by using a single compensation pixel for each defective pixel.20. The method according to claim 13 or 15, wherein said compensationfor said at least one defective pixel is performed by using a pluralityof compensation pixels for each defective pixel.
 21. The methodaccording to claim 13 or 15, wherein said at least one SLM isilluminated by the same radiation dose in different writing passes. 22.The method according to claim 13 or 15, wherein said at least one SLM isilluminated by different radiation dose in different writing passes. 23.The method according to claim 13 or 15, wherein said at least one SLM isa transmissive Spatial Light Modulator.
 24. The method according toclaim 13 or 15, wherein said at least one SLM is a reflective SpatialLight Modulator.
 25. The method according to claim 13 or 15, wherein thepixels in said at least one SLM is operated in an analog manner.
 26. Themethod according to claim 13 or 15, wherein an image of said pixels insaid first writing pass is displaced in said at least one SLM relativesaid image of said pixels in said second writing pass by one or aplurality of pixels.
 27. The method according to claim 13 or 15, whereinan image of said pixels in said first writing pass is displaced on saidworkpiece relative said image of said pixels in said second writing passby at least a fraction of a pixel.
 28. The method according to claim 13or 15, wherein said first set of pixels belong to a first SLM and saidsecond set of pixels belong to a second SLM.
 29. The method according toclaim 28, wherein said first and second SLM:s are illuminatedsimultaneously.
 30. The method according to claim 29, wherein said firstand second SLM:s are illuminated by different radiation intensities. 31.A method for compensating the impact of at least one defective pixelwith a known position in at least one spatial light modulator (SLM) whencreating a pattern of said at least one SLM on a work piece covered witha layer sensitive to electromagnetic radiation, comprising the actionsof: fracturing and rendering the pattern to be printed, calculatingexposure values for at least one SLM pixels, locating bad pixels,determine if said bad pixels are in a critical position, providing asource for emitting electromagnetic radiation, illuminating by saidradiation said at least one SLM having a plurality of modulatingelements (pixels), compensating for bad pixels in critical positions.32. An apparatus for compensating the impact of at least one defectivepixel with a known position in at least one spatial light modulator(SLM) when creating a pattern of said at least one SLM on a work piececovered at least partly with a layer sensitive to electromagneticradiation, comprising a source to emit electromagnetic radiation, aprojection system to project in a first writing pass an image of saidmodulator on said work piece using a first set of pixels in said atleast one SLM, at least one pixel in said at least one SLM in at leastone prior writing pass to perform a pre compensation of defective pixelsin at least one subsequent writing pass, a projection system to projectin at least a second writing pass said image of said modulator on saidworkpiece using at least a second set of pixels in said at least oneSLM, at least one pixel in said at least one SLM in at least one latterwriting pass to perform a post compensation of defective pixels in atleast one prior writing pass.
 33. The apparatus according to claim 32,wherein said electromagnetic radiation is a pulsed laser source.
 34. Theapparatus according to claim 32, wherein at least one pixel in saidfirst set of pixels is included in said at least a second set of pixels.35. The apparatus according to claim 32, wherein said projection systemto project in at least a second writing pass comprises said SLMreprogrammed with the image to be written on said work piece with saidat least second set of pixels, a movable stage upon which stage saidwork piece is arranged in order to illuminate the same feature on saidwork piece with said at least second writing pass as said first writingpass.
 36. The apparatus according to claim 32, wherein said movablestage is moved the length of N SLM pixels.
 37. The apparatus accordingto claim 36, wherein said stage is moved along a row of pixels.
 38. Theapparatus according to claim 36, wherein said movable stage is movedalong a column of pixels.
 39. The apparatus according to claim 36,wherein said movable stage is moved along both a row of pixels and acolumn of pixels.
 40. The apparatus according to claim 32, wherein saidmovable stage is moved the length of N SLM pixels plus a fraction of aSLM pixel.
 41. The apparatus according to claim 32, wherein a singledefective pixel is compensated in one writing pass with a singlecompensating pixel in another writing pass.
 42. The apparatus accordingto claim 32, wherein a single defective pixel in one writing pass iscompensated with a plurality of compensating pixel in another writingpass.
 43. The apparatus according to claim 32, wherein said at least oneSLM is illuminated by a same radiation dose in the different writingpasses.
 44. The apparatus according to claim 32, wherein said at leastone SLM is illuminated by different radiation intensities in thedifferent writing passes.
 45. The apparatus according to claim 32,wherein said at least one SLM is a transmissive Spatial Light Modulator.46. The apparatus according to claim 32, wherein said at least one SLMis a reflective Spatial Light Modulator.
 47. The apparatus according toclaim 32, wherein the pixels in said at least one SLM is operated in ananalog manner.
 48. An apparatus for compensating the impact of at leastone defective pixel with a known position in at least one spatial lightmodulator (SLM) when creating a pattern of said at least one SLM on awork piece covered at least partly with a layer sensitive toelectromagnetic radiation, comprising a source for emittingelectromagnetic radiation, a projection system for illuminating said atleast one SLM, having a plurality of modulating elements (pixels), bysaid radiation and projecting in a writing pass an image of saidmodulator on said work piece, a detector arrangement (65) for measuringthe dose of pixels from the image of said at least one SLM and acomputer (66) for performing a compensation for defective pixels in atleast one other writing pass out of said image on said detector (65).49. An apparatus for compensating the impact of at least one defectivepixel with a known position in at least one spatial light modulator(SLM) (30) when creating a pattern of said at least one SLM (30) on awork piece (60) covered at least partly with a layer sensitive toelectromagnetic radiation, comprising a source for emittingelectromagnetic radiation, a projection system for illuminating said SLM(30), having a plurality of modulating elements (pixels), by saidradiation and projecting in a writing pass an image of said modulator(30) on said work piece (60), a detector arrangement (65) for measuringthe dose of pixels from the image of said at least one SLM, and acomputer (66) for performing a compensation for defective pixels (110)by using at least one of the most adjacent pixels (111, 112, 113, 114,115, 116, 117, 118) to said defective pixel (110).
 50. The apparatusaccording to claim 48 or 49, wherein the pixel intensities is detectedby said detector arrangement (65) whenever a new work piece (60) is tobe patterned.
 51. A method for detecting at least one defective pixel inat least one SLM, comprising the actions of addressing all pixels insaid at least one SLM with a first steering signal, illuminating said atleast one SLM with electromagnetic radiation, recording an image of saidat least one SLM, computing a gradient field of the recorded image,computing a divergence of the gradient field, identifying extreme valuesfrom the computed divergence which corresponds to defective pixels. 52.The method according to claim 51, further comprising the actions ofaddressing all pixels said at least one SLM with a second steeringsignal, illuminating said at least one SLM with electromagneticradiation, recording an image of said at least one SLM, computing agradient field of the recorded image, computing a divergence of thegradient field, identifying extreme values from the computed divergence,where defective pixels corresponds to extreme values from said firststeering signal and said second steering signal representing samepixels.
 53. A method for detecting at least one defective pixel in atleast one SLM, comprising the actions of addressing all pixels in saidat least one SLM with a first steering signal, illuminating said atleast one SLM with electromagnetic radiation, recording a first image ofsaid at least one SLM, addressing all pixels in said at least one SLMwith a second steering signal, illuminating said at least one SLM withelectromagnetic radiation, recording a second image of said at least oneSLM, computing the difference between said first image and said secondimage, identifying bad pixels where the computed difference has a localminimum.
 54. A method for detecting at least one defective pixel in atleast one SLM, comprising the actions of addressing a pattern to said atleast one SLM, illuminating said SLM with electromagnetic radiation,recording a first image of said at least one SLM, comparing saidrecorded image with pattern data at feature edges, identifying badpixels where the feature edge is moved a predetermined distance.
 55. Themethod according to claim 54, wherein said pattern is a chessboardpattern.
 56. The method according to claim 54, wherein said pattern is apattern with parallel lines.
 57. The method according to claim 54,further comprising the action of addressing said pattern with anotherset of pixels in said at least one SLM, illuminating said SLM withelectromagnetic radiation, recording a second image of said at least oneSLM, comparing said recorded second image with pattern data at featureedges, identifying bad pixels where the feature edge is moved apredetermined distance.
 58. The method according to claim 57, furthercomprising the action of comparing the feature edge movement in saidfirst image with said second image for identifying bad pixels stuck atintermediate values.
 59. The method according to claim 5, wherein saiddifferent writing passes is performed by means of one SLM.
 60. Themethod according to claim 59, wherein different areas of said SLM areused in the different writing passes.
 61. The method according to claim5, wherein said different writing passes is performed by means of aplurality of SLMs.
 62. The method according to claim 6, wherein saiddifferent writing passes is performed by means of one SLM.
 63. Themethod according to claim 62, wherein different areas of said SLM areused in the different writing passes.
 64. The method according to claim6, wherein said different writing passes is performed by means of aplurality of SLMs.